This is a classic, and when you watch a classic you assume it's going to be good. And often they are, except a few hiccups or things that feel dated. But often its "classic" status is doing a lot of the work for your appreciation of it.
But this movie is WOW. It is not still heralded only because of its past accolades. The performances are so powerful that it feels fresh. Both parts and both women are amazing. I alternated throughout thinking "Joan Crawford got the better role...no, Bette Davis got the better role..."
Even though the film runs longer than two hours, it has what so few films today lack--economy. Scenes are not wasted. Every scene moves the plot forward or provides the motivation for a character. But at the same time, points are not belabored. Some things are telegraphed. For example, Baby Jane's vaudeville billing calls her "from Duluth." Nothing is ever said about this, but that small fact telegraphs so much about the family's background and how they respond to Baby Jane's talent/fame. That's just one example of ways the movie telegraphs. Some things were Bette Davis's idea, like how Jane does her makeup, and they're just genius. Again, a small thing that just communicates so much.
It's a real powerhouse. Madness, manipulation, half-truths, cruelty both accidental and calculated.
If it's been on your list for a while but you haven't seen it, watch it.